Heat exchanger for thermoelectric applications

ABSTRACT

A thermoelectric system ( 10 ) for pumping heat having at least one foam heat exchanger ( 45 ) is provided that enhances heat transfer away from the system ( 10 ) to increase overall system efficiency and performance of the system.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to foam heat exchangers, and moreparticularly, to an apparatus and method for enhancing heat transfer inthermoelectric systems using foam heat exchangers.

2. Description of Related Art

The use of heat exchangers to dissipate heat in power electronicsapplications is well known. Heat exchangers or heat sinks are frequentlymetal radiators designed to remove heat from power electronicscomponents, particularly, power transistor modules, by thermalconduction, convection or radiation. Without heat exchangers powerelectronics component would suffer from reduced performance andreliability.

Heat exchangers are often structured to have a maximum number of finsper unit volume radiating in a direction perpendicular to a heatedsurface. In particularly demanding applications, heat exchangersdissipate heat using forced convection to a cooling fluid over the heatexchangers to increase the heat dissipation of the exchanger. An evenmore efficient apparatus for dissipating heat is the use of foams, andin particular metal forms, which have a more effective surface area forheat transfer. Metal foams have recently been used to dissipate heat inpower electronic applications; however, they have not been used inthermoelectric systems.

Accordingly, there exists a need for foam heat exchangers to be usedwith thermoelectric elements to build systems for a variety of heatingand cooling systems that reduce energy consumption and increase heatpumping capacity in such systems.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide thermoelectricheating and cooling systems that use foam heat exchangers.

It is also an object of the present invention to provide thermoelectricheating and cooling systems that use metal foam heat exchangers.

It is another object of the present invention to provide thermoelectricheating and cooling systems that use foam heat exchangers to dissipateheat.

It is a yet another object of the present invention to providethermoelectric heating and cooling systems having thermoelectricelements that use foam heat exchangers to reduce the energy consumptionof the thermoelectric elements.

It is still yet another object of the present invention to providethermoelectric heating and cooling systems having thermoelectricelements that use foam heat exchangers to increase the heat pumpingcapacity of the thermoelectric elements.

It is a further object of the present invention to provide a method forenhancing heat transfer of thermoelectric elements using foam heatexchangers.

A system for enhancing the efficiency of a thermoelectric heat pumpingsystem including an array of thermoelectric elements having atemperature at a first surface of the array and a temperature at asecond surface of the array opposite the first surface and at least onefoam heat exchanger located adjacent one of the first surface and thesecond surface is provided. The fluid flowing through the at least onefoam heat exchanger reduces a difference between the temperature at afirst surface of the array and the temperature at a second surface ofthe array thereby enhancing the efficiency of the system.

A method of enhancing the efficiency of a thermoelectric system having athermoelectric array having a series of thermoelectric pairs arrangedelectrically in series is provided. The method provides for a first foamheat exchanger adjacent a first surface of the thermoelectric array anda second foam heat exchanger adjacent a second surface of thethermoelectric array opposite first surface; for generating atemperature at a first surface of the thermal array and a temperature ata second surface of the array that is different from the temperature atthe first surface of the array; whereby fluid flowing through the firstfoam heat exchanger and the second foam heat exchanger reduces atemperature difference between the first surface and the second surface,thereby enhancing the efficiency of the thermoelectric system.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a thermoelectric system having foam heat exchangersof the present invention;

FIG. 2 shows a table that compares the heat transfer coefficients ofdifferent foams used in the thermoelectric system of the presentinvention and the weight savings compared to a conventional heatexchanger;

FIG. 3 illustrates a thermoelectric system functioning in a heating modeand using foam heat exchangers of the present invention;

FIG. 4 illustrates a graph showing increased coefficient of performanceof thermoelectric systems as the heat transfer coefficient of heatexchangers increase;

FIG. 5 illustrates a foam heat exchanger of the present invention shownin FIG. 3; and

FIG. 6 illustrates a foam heat exchanger according to a secondembodiment of the heat exchanger of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIG. 1, a thermoelectric system 10 having a thermoelectricelements 15, is shown. Thermoelectric elements 15 are grouped in severalP and N pairs or couples 20 that are arranged electrically in series.Electrical connectors 25 provide the connection between adjacent couples20 and to a power source (not shown). Substrates 30 and 35 are ceramicsubstrates that provide insulation to system 10. Substrates 30 and 35hold system 10 together mechanically and insulate couples 20electrically. Substrate 30 has a surface 40 that is in contact a withfoam heat exchanger 45. Similarly, substrate 35 has a surface 50 that isin contact with foam heat exchanger 55. Fans 60 and 65 are used to forcefluid through heat exchangers 45 and 55, respectively. Although fans 60and 65 are shown forcing air through heat exchangers 45 and 55,respectively, other types of mechanisms for removing other types offluid could also be used. Surfaces 40 and 50 may be integral to heatexchanger 45 and 55, respectively, and form a base for connecting tosurface 30 and 35 of thermoelectric array.

In FIG. 1, foam heat exchangers 45 and 55 are located immediatelyadjacent to substrates 30 and 35 to maximize heat transfer from thesurfaces 70 and 75 of thermoelectric elements 15. Foam heat exchangers45 and 55 provide enhanced heat transfer area from surfaces 70 and 75,respectively.

Foam heat exchangers 45 and 55 are made from highly conductive materialssuch as aluminum, copper or graphite. Exchangers made from suchmaterials are not only highly conductive, but because they are formed asa foam, they have a very high porosity and surface area to furtherenhance their heat transfer capacity. Traditional heat exchangers usedin thermoelectric applications have fins to dissipate heat. Incomparison to foam heat exchanges, finned heat exchangers have a verylimited surface area. Furthermore, traditional heat exchangers arerelatively heavy compared to foam heat exchangers 45 and 55 of thepresent invention. Reducing the weight and/or volume and increasing theheat transfer capacity of heat exchangers is of great concern when bothsmall and large heating and cooling thermoelectric systems are used.

Referring to Table 1 in FIG. 2, the heat transfer coefficients, maximumtemperature and weight savings of a traditional heat sink compared tothree foam heat exchangers of differing porosities, is shown. ComparingFoam A having a porosity of 10 PPI (pores per inch), the coefficient ofheat transfer is over eighty-seven (87) times greater that that of thetraditional heat sink. By doubling the porosity of the foam heatexchanger to 20 PPI, the coefficient of heat transfer of the Foam B isincreased to one hundred and thirty (130) times that of the conventionheat sink. Again doubling the porosity of the foam heat exchanger to 40PPI, the coefficient of heat transfer of the Foam C is increased to onehundred and eighty-eight (188) times that of the convention heat sink.Not only is there a tremendous increase in heat transfer capacity, butthe weight savings of the foam heat exchangers is also significant. Thesubstantial weight savings reduces the overall weight of thethermoelectric refrigeration or heating system is which these exchangerswould be used. Further, by reducing the maximum temperature of thesystem, the overall temperature difference across the thermoelectricarray is decreased significantly. The coefficient of performance (COP)of thermoelectric systems is defined as the heating or cooling capacitydivided by the power consumed. The COP is inversely proportional to themaximum temperature difference across the array.

Referring to FIG. 3, a first embodiment of the present invention havinga thermoelectric system 90 using foam heat exchangers 95 and 100configured in a heating mode, is shown. A DC voltage from a power source105 is applied across thermoelectric elements 120 and a current 110flows in the direction shown. Pairs 115 (P and N pairs) ofthermoelectric elements 120 absorb heat from a surface 125 and releaseheat to a surface 130 at the opposite side of device 120. Surface 125where the heat energy is absorbed becomes cold and the opposite surface130 where the heat energy is released becomes hot. This “heat pumping”phenomenon, known as the Peltier effect, is commonly used inthermoelectric refrigeration or heating. In this embodiment, fan 135forces air through heat exchanger 100 which absorbs heat and is cooled.Fan 140 forces air through heat exchanger 95 to transport heat away fromsurface 130 to be heated. Power source 105 used in this configurationcan be a battery, a fuel cell or any other similar device used to supplycurrent. Thermoelectric system 90 can be converted from a heating modeto a cooling by reversing the polarity of DC poser supply 105.

Foam heat exchangers 95 and 100 provide substantial heat transfercapacity across surfaces 130 and 125, respectively, compared totraditional heat sinks to increase the efficiency of system 90. Byhaving a high heat transfer coefficient foam heat exchangers 95 and 100,a lower the temperature difference between the opposing surfaces ofthermoelectric elements 120, is achieved. This low temperaturedifference increases the performance of the overall system 90 byconsuming less energy. Thus the overall system, whether it is configuredas a heating or a cooling system, has a very high performance.

FIG. 4 shows the relationship between performance of the system and thecoefficient of heat transfer using the foam heat exchangers of a typicalthermoelectric system. Coefficient of performance is defined as heatingor cooling capacity divided by the power consumed by the system.

Referring to FIG. 5 a second configuration of a foam heat exchangersystem 150 is shown. Foam heat exchanger system 150 has a thermoelectricarray 155 having a series of thermoelectric pairs 160 arranged inseries. Thermoelectric device array 155 has surfaces 165 and 170. System150 is arranged to have a single foam heat exchanger 175 to dissipateheat from surface 170. Depending upon the application, a second foamheat exchanger may not be required. Alternatively, a traditional heatexchanger may be used in place of a foam heat exchanger depending uponthe application and placed adjacent surface 165. Differentconfigurations of placing foam heat exchangers can be used to maximizeheat transfer and depending upon the application. Similarly, a singlesystem can include several thermoelectric array, each having one or morefoam heat exchangers.

In FIG. 6 a third embodiment of a foam heat exchanger system 180, isshown. System 180 is arranged similar to the system of FIG. 5, exceptthat the heat exchanger is a combination foam and fin heat exchanger185. System 80 has an array 190 of thermoelectric elements 195. Elements195 have surfaces 200 and 205. In the embodiment of FIG. 5, a secondfoam heat exchanger may not be required. Alternatively, a traditionalheat exchanger may be used in place of a foam heat exchanger dependingupon the application. Additionally, different configurations of placingfoam heat exchangers can be used to maximize heat transfer and dependingupon the application.

While the instant disclosure has been described with reference to one ormore exemplary embodiments, it will be understood by those skilled inthe art that various changes may be made and equivalents may besubstituted for elements thereof without departing from the scopethereof. In addition, many modifications may be made to adapt aparticular situation or material to the teachings of the disclosurewithout departing from the scope thereof. Therefore, it is intended thatthe disclosure not be limited to the particular embodiment(s) disclosedas the best mode contemplated for carrying out this invention, but thatthe invention will include all embodiments falling within the scope ofthe appended claims.

1. A system for enhancing the efficiency of a thermoelectric heatpumping system comprising: an array of thermoelectric elements having atemperature at a first surface of said array and a temperature at asecond surface of said array opposite said first surface and a foam heatexchanger adjacent a first surface of said array and a foam heatexchanger adjacent a second surface of said array wherein fluid flowingthrough said foam heat exchanger adjacent said first surface and fluidflowing through said foam heat exchanger adjacent a second surface ofsaid array reduce a difference between said temperature at a firstsurface of said array and said temperature at a second surface of saidarray thereby enhancing the efficiency of said system.
 2. (canceled) 3.The system of claim 1, comprising a current flowing through said arrayof thermoelectric elements to generate a temperature difference betweena first surface of said array and a second surface of said array.
 4. Thesystem of claim 3, wherein said at least one foam heat exchanger at oneof said first surface of said array and said second surface of saidarray transport heat away from said array thereby reducing the currentflowing through said array.
 5. The system of claim 1, wherein said foamheat exchanger adjacent said first surface of said array and said foamheat exchanger adjacent said second surface of said array each have aporosity to enhance heat transfer through said array.
 6. The system ofclaim 1, wherein said at least one foam heat exchanger incorporates finsfor heat dissipation.
 7. The system of claim 1, wherein said at leastone foam heat exchanger is made from a material selected from a groupconsisting of aluminum, graphite and copper.
 8. (canceled)
 9. A methodof enhancing the efficiency of a thermoelectric system comprising:providing a thermoelectric array having a series of thermoelectric pairsarranged electrically in series; providing a first foam heat exchangeradjacent a first surface of said thermoelectric array and a second foamheat exchanger adjacent a second surface of said thermoelectric arrayopposite said first surface; generating a temperature at said firstsurface of said thermal array and a temperature at a second surface ofsaid array that is different from said temperature at said first surfaceof said array; whereby fluid flowing through said first foam heatexchanger and said second foam heat exchanger reduce a temperaturedifference between said first surface and said second surface, therebyenhancing the efficiency of the thermoelectric system.
 10. The method ofclaim 9, wherein said first foam heat exchanger and said second foamheat exchanger each have a porosity to enhance heat transfer capability.11. The method of claim 9, wherein as a temperature between said firstsurface and said second surface is reduced, a coefficient of performanceof said system is increased.
 12. The method of claim 9, wherein areduced porosity of said first foam heat exchanger and said second foamheat exchanger further enhance heat transfer to or away from said firstsurface and said second surface.
 13. The method of claim 9, whereinenhanced heat transfer across said first surface and said second surfacereduces required current flowing through said array.
 14. The method ofclaim 9, wherein at least one of said first foam heat exchanger and saidsecond foam heat exchanger incorporate fins.
 15. (canceled)